Ed Pollack does a bit of tuning:
SQL Server provides a variety of ways to tune XML so that it provides consistent performance, consumes less space, all while ensuring efficient access to critical data.
At its core, the metadata-styled XML format runs counter to the data that SQL Server is optimized to manage. Therefore, additional features were added to SQL Server over time that allowed for XML data to be indexed and compressed.
While these features are critical for managing XML data as it becomes large, it is important to remember what XML is intended for and why it is (loosely) structured as it is. Many data professionals have used shortcuts when XML was small, such as storing and analyzing it in string format, only to be forced to reckon with performance challenges when scanning large strings become agonizingly slow.
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